1. Field of the Invention
This invention relates to an exhaust gas recirculation control system used for removing nitrogen oxides (NOx) in exhaust gases discharged from internal combustion engines.
2. Description of the Priot Art
In conventional exhaust gas recirculation control systems hitherto used, an exhaust gas recirculation control valve for controlling the recirculation gas flow is generally operated by intake negative pressure introduced thereinto from a throttle valve of a carburetor through a negative pressure passage including in its midway a negative pressure regulator which is operated by an exhaust gas pressure to regulate the intake negative pressure in order to maintain an optimum amount of the recirculation gas flow according to a condition of the engine operation.
With this arrangement of the prior art, however, the exhaust gas recirculation control valve is apt to close prematurely when the throttle valve is widely opened, for example, in accelerating the engine, so that the nitrogen oxides in the exhaust gases cannot be purified. Moreover, when the engine is accelerated, the nitrogen oxides in the exhaust gases are much more than those during normal travelling. Accordingly, it is necessary to increase the recirculation gas flow when accelerating. The control systems of the prior art are not sufficient to achieve a complete purification of the nitrogen oxides in the exhaust gases.